A portable modular optical sensor capable of measuring complex multi-axis strain fields

Abstract

This paper presents a portable optical sensor capable of measuring complex multi-axis strain fields without the need for special surface preparation or stringent sensor-to-surface alignment. The sensor consists of three to four electronic speckle photography (ESP) modules. The design of each modular element is based on a previously developed 5-axis (five degree of freedom) surface displacement measurement technique, and is able to measure two dimensional in-plane surface movement, unaffected by other degrees of freedom (displacement and rotation) movement. Identical modular strain elements are arranged in a Rosette grid layout so that accurate and robust multi-axis surface strain measurement can be achieved. Experiments were conducted to demonstrate the multi-axis strain field measurement capability of this optical sensor by using a test bed that provided a known directional planar strain field, and excellent results were obtained. In particular, experiments have shown that the principle strain can be accurately extracted independent of the orientation of the device. This portable optical sensor will allow precise non-contact measurement of practical complex strain fields such as those encountered in bridge abutments, and portions of beams near critical infrastructure support locations; in other words, wherever plane strains depart from uni-axial behavior. Its unique hand-held portable capability offers distinct advantages over laboratory strain measurement setups, allowing accurate robust non-contact measurements to be achieved even in a harsh field application environment.